The Ghana Grid Company Limited (GRIDCo) has explained that the power system experienced a total collapse when the Prestea-Obuasi transmission line tripped on a fault while Bui was running in synchronous condenser mode.
In its latest report on the system collapse, GRIDCo explained that loading on the Aboadze-Anwomaso 330 kV transmission line (TT8AW) increased causing overloading on the Anwomaso 330/161 kV transformers.
The transformers tripped together with the TT8AW line and caused a severe surge on the NITS.
This led to cascaded trips on other transmission lines and generating units in service, leading to a total system collapse at 14:04 hours. Restoration started immediately and by 18:30 h, supply had been restored to all customers.
Prior to the system collapse, the power system was in normal operating condition. The state of the generation and transmission system prior to the collapse was as follows:
i. Generation
Four (4) generating units were in service at Akosombo, three (3) units at Kpong GS, one (1) in synchronous condenser mode at Bui, three (3) units each at TAPCO and TICO, one (1) unit at KTPP, two (2) units each at Cenpower and Amandi, seven (7) units at Sunon Asogli and twenty-four (24) units at Karpower. The total system generation was 2,315 MW.
ii. Transmission
All transmission lines were in service at the time of the incident except the Volta-Achimota transmission line No. 2 (V19H) which was out of service to provide safe working space to erect a tower for the new Volta-Accra East-Achimota transmission corridor upgrade project.
According to the report, the voltage at Volta was 160 kV. The Ghana-Cote d’Ivoire interconnection was in service with inadvertent export of 39.9 MW to CIE. Power exports were as follows: SONABEL: 45.6 MW and CEB: 67.7 MW.
At 13:46 hours, the Prestea-Obuasi (P6B) transmission line tripped on a fault.
While the operator at SCC was making efforts to restore the line, loading on the Aboadze-Anwomaso 330 kV transmission line (TT8AW) increased to 450 MW.
This caused overload on the Anwomaso 330/161 kV auto transformers (58T3 and 58T4). At 13:48 h, 58T3 and 58T4 transformers tripped on overload, causing an opening of TT8AW line at Anwomaso end. The TT8AW line tripped at Aboadze end on overvoltage when the Anwomaso end opened.
The tripping of the P6B and TT8AW lines resulted in a surge that caused all generating units in service at TAPCO, TICO, Karpower, Cenpower and Amandi to trip. At 14:04 h, all Akosombo and Kpong units tripped leading to a total system collapse.
However, restoration was started immediately after the collapse with Akosombo unit No. 2 tied to the system, followed by unit No. 4.
Supply was restored to VALCO, followed by Achimota, New Tema and Smelter II. Units were brought online at Karpower and Sunon Asogli and supply was restored to Accra East, Tafo, Nkawkaw, Anwomaso and Kumasi.
Supply was restored progressively as more generation came online and by 18:30 h, supply to all customers on the NITS had been restored.
The report recommended that ongoing work on the Anwomaso-Kintampo 330 kV transmission line should be expedited since the completion of the line will reduce loading on the Anwomaso 330/161 kV auto transformers and improve system stability during contingencies.
Story: News Desk
